1. Field of the Invention
This invention relates in general to methods for testing magnetoresistive sensors, and more particularly to a method and apparatus for quantifying stress and damage in magnetic heads.
2. Description of Related Art
Magnetic storage systems, such as magnetic disk storage systems, are commonly used to store digital information. There has been an ongoing effort to reduce the size of such magnetic storage systems while increasing their storage capacity. This has led to components that are smaller yet, which must provide ever-increasing performance. The disk drive includes a mechanical portion in the form of a head-disk assembly and an electronics portion in the form of a printed circuit board assembly that controls functions of the head-disk assembly while providing a communication interface to a host being serviced by the disk drive.
The head-disk assembly has a disk with a recording surface rotated at a constant speed by a spindle motor assembly and an actuator assembly positionably controlled by a closed loop servo system for use in accessing the stored data. The actuator assembly supports a magnetoresistive head with an inductive element, or writer, to write data to and a magnetoresistive (MR) element, or reader, to read data from the recording surface. Such magnetoresistive elements have in the past relied on the anisotropic magnetoresistive (AMR) effect, in so-called AMR sensors, but contemporary magnetoresistive elements rely on a giant magnetoresistive (GMR) effect, in so-called GMR sensors.
The disk drive market continues to place pressure on the industry for disk drives with increased capacities, higher data rates and lower costs. The magnetoresistive head is a high cost component of the disk drive. The trend toward smaller component size and increased component performance has resulted in an increased likelihood that a component could fail. As each head passes through manufacturing processes in preparation for use in a disk drive, costs associated with those processes accrue and contribute to the overall cost of the disk drive. Early detection of potential failure, preferably during manufacture of the storage system, can increase the reliability of those system products that are placed into use.
One type of failure is related to the transducer heads used in such magnetic storage systems. Transducer heads are used to read and write information on a magnetic storage medium. By measuring characteristics of the head throughout the manufacturing process, defective and marginally defective heads can be culled from the process before additional costs are needlessly applied. However, measuring the ability of a magnetic head to withstand inadvertent and often damaging head/disk interactions (HDI's), a term well known in the art of magnetic recording, is an imprecise science. Certain head designs and materials are more or less susceptible to head/disk interaction (HDI) damage, and it is important to know which design is more robust in the drive and less susceptible to damage. Choosing the correct design and material set requires a means to measure and assess the effects of damage caused by head/disk interactions.
One method for assessing damage caused by head/disk interactions involves writing a special bit pattern on a disk using a wide writing head. The polarity of the read-back signal from this pattern is observed as the head to be measured traverses a portion of the medium with asperities and mechanical irregularities, a rough zone. Due to mechanical interactions, so-called head/disk interactions (HDI's), with the asperities on the disk, various performance parameters of the head can degrade amongst these, in particular, asymmetry for which a change in the sign or polarity of the read-back signal might reverse, indicating damage to the read sensor.
For example, a giant magnetoresistive (GMR) read sensor includes a sandwich of layers, also known as a sensor stack, including a ferromagnetic pinned layer (PL), a nonmagnetic electrically conducting layer, and a ferromagnetic free layer (FL). Such GMR sensors include so-called spin valve sensors of the current-in-plane (CIP) and current-perpendicular-to-plane (CPP) variety, tunneling magnetoresistive (TMR) sensors, as well as hybrid type sensors having both TMR and CPP GMR aspects in their construction. The resistance of the magnetoresistive (MR) or giant magnetoresistive (GMR) sensor changes with respect to the direction and magnitude of an applied magnetic field such as the field from a written magnetic transition on a disk. To detect the change in resistance, sense current is passed through the sensor through electrical leads. Typically, hard bias material is disposed in layers near the ends of a sensor stack forming permanent magnets, which impose a stabilizing magnetic biasing field on the sensor stack.
All GMR sensors, and particularly self-biased GMR sensors, are subject to a reversal of the direction of magnetization in the pinned layer. A magnetization reversal occurs when the direction of magnetization in the pinned layer is rotated approximately 180 degrees. A sensor that has experienced magnetization reversal in the pinned layer will exhibit a polarity reversal in the read-back signal. Thus, the read-back signal from a written transition that was originally positive will become negative if a polarity reversal in the pinned layer has occurred. Typically, the recorded information of the servo system is polarity sensitive. In some applications, the synchronization field recorded on the data track is also polarity sensitive. Accordingly, a disk drive having a magnetoresistive sensor that has undergone a magnetization reversal in the pinned layer will no longer function properly. A disk drive user may no longer be able to access the data stored on the disk drive.
While detecting polarity reversal is theoretically sound, the technique is somewhat difficult to reduce to practice because the technique requires a bit pattern to be imparted to the test medium with a wide writing head that may not be readily available and polarity reversal of the read-back signal from the written pattern is not easily detected. Moreover, this technique requires a medium with a previously written bit pattern.
It can be seen then that there is a need for a method and apparatus for quantifying stress and damage in magnetic heads.